1. Field of the Invention
The present invention relates to the field of power station technology. It refers to a generator cooling system for a generator used for current generation in a power station, said generator having a generator cooler which is arranged, together with further coolers, in a closed intermediate cooling circuit which transfers heat to a main cooling water system via at least one intermediate cooler.
2. Discussion of Background
Generators used for current generation in a power station have to be cooled in order to discharge the energy loss heat occurring during operation. In this case, a distinction is made, according to the cooling system, between open-cooled generators and closed-cooled generators. Open-cooled generators operate with air cooling. In closed-cooled generators, air cooling, hydrogen (H.sub.2) cooling, water cooling or mixed cooling, in which the rotor is cooled by hydrogen and water and the stator by water, may be used.
For the maximum apparent power output which a generator can deliver depends heavily on the temperature of the components, that is to say on the temperature of the cooling medium. The lower the temperature of the heat sink, into which the energy loss heat is discharged, the higher the maximum operable apparent power output of the generator, or the longer the life of the generator in the case of a predetermined fixed apparent power output. If appropriate, it is then also possible to change over from complex costly generator cooling to simpler cheaper generator cooling, for example from hydrogen cooling to air cooling.
An example of a known generator cooling system is illustrated in FIG. 1. The generator cooling system 10 comprises a closed intermediate cooling circuit 18, in which a cooling medium, usually water, circulates. Connected in parallel in the intermediate cooling circuit 18 are a generator cooler 11 with a plurality of individual coolers 111 to 114 and, as further coolers, for example an oil cooler 12, a boiler pump cooler 13, two feed water pump coolers 14 and 15, a sampling cooler 16 and a fuel gas compressor cooler 17. The cooling water is pumped through the intermediate cooling circuit 18 by two cooling water pumps 26 and 27 working in parallel. It flows through two arranged in parallel, intermediate coolers 19 and 20, through which the main cooling water of the main cooling water system (34 in FIG. 6) flows. The main cooling water system is not illustrated in FIG. 1. The connection to this system is symbolized by an inlet 21 and an outlet 23 for the main cooling water. A water filter 22 may be arranged upstream of the intermediate coolers 19, 20 in the main cooling water system. Furthermore, a metering device 25 for a protective agent (inhibitor) may be connected in parallel to the cooling water pumps 26, 27, said metering device being supplied with a suitable protective agent via an inlet 24. Moreover, the closed intermediate cooling circuit may be filled up with additional water via an inlet 28. Finally, the circuit also has to be connected to an equalizing tank 29. The individual plant parts are in each case equipped with valves which are illustrated in the figures by corresponding symbols, but, for the sake of simplicity, are not given reference marks.
An example of a suitable main cooling water system 34 is illustrated in FIG. 6. In this system, the intermediate coolers 19, 20 are arranged parallel to a main condenser 35 of the power station. The heated main cooling water is cooled in a cooling tower 36 which is equipped with a cooling tower fan 37, collects via a return 38 in a collecting pond 39 and is pumped back from there to the cooling points 19, 20 and 35 by means of two parallel main cooling water pumps 40, 41. The water evaporating in the cooling tower 36 is supplemented by means of an inlet 30 for additional cooling tower water. With regard to throughflow cooling, the cooling tower 36 may also be replaced by a water reservoir, for example a river, a lake or the ocean.
In the system according to FIGS. 1 and 6, the generator cooler 11 cools the cooling medium (H.sub.2, air, water) used for cooling the generator. In this case, the temperature of the cooling medium can be influenced by the temperature of the heat sink (in the conventional case of the main cooling water), by the temperature difference ratings of the heat exchangers (coolers) used and by the mass flow conditions in the cooling circuits. Whereas, in the past, the performance of generators was more than adequate, as compared with plant output (for example, the gas turbines used), and high power reserves were available in the entire outside air temperature range (in the case of cooling tower cooling) or water temperature range (in the case of throughflow cooling), nowadays the cooling of generators is coming up against its limits on account of the rise in plant output, the increasing cost pressure, the limitation of the output of air-cooled generators to 300-350 MW at the present time, etc. However, in the case of smaller plants, too, or, for example, in the retrofit business, efficient generator cooling may assume critical importance.
Consequences arising from this situation would be:
a transition from air cooling to H.sub.2 cooling PA1 that there would be a jump in power output and therefore cost PA1 restrictions in the power factor (cos .phi.) PA1 that the prescribed insulation class would be overstepped by a few degrees (for example, B+5K) PA1 a reduction in the life of the generator.
Another reason for improving generator cooling may also be, for example, an increase in the power output of an existing plant as a result of process improvements. This avoids the need for a generator replacement.